This invention relates to an adaptive equalizer which is used in combination with a demodulator.
Into a demodulated signal, the demodulator demodulates a received signal which may be susceptible to fading while transmitted to the demodulator through a radio channel. The demodulated signal may be a baseband signal having a binary level or a multilevel.
A conventional adaptive equalizer is supplied with the demodulated signal and comprises a modifying circuit, an equalizer section, an expanding circuit, and a control circuit which will presently be described.
The modifying circuit is for carrying out a compression operation on the demodulated signal in accordance with a compression ratio and produces a modified signal representative of a result of the compression operation. The equalizer section is for carrying out an equalization operation on the modified signal in accordance with controllable tap gains and produces an equalized signal representative of a result of the equalization operation. The expanding circuit is for carrying out an expansion operation on the equalized signal in accordance with an expansion ratio and produces an expanded signal representative of a result of the expansion operation. The control circuit is for controlling each of the compression and the expansion ratios in accordance with the controllable tap gains.
The fading has a magnitude which varies dependent on conditions of the radio channel. When the magnitude of the fading becomes greater than a predetermined value, the control circuit makes each of the compression and the expansion ratios change stepwise. In this connection, a time lag is inevitably between a time instant when the compression ratio is changed and another time instant when the expansion ratio is changed. This results in malfunction of the equalization operation in the equalizer section. This is because the equalizer section does not exactly carry out the equalization operation during a particular time interval which is determined due to the time lag.
In addition, each of the compression and the expansion ratios is determined in accordance with the controllable tap gains. In this connection, each of the compression and the expansion ratios is given an incoherent value when the malfunction is caused in the equalizer section to make the controllable tap gains become abnormal. When at least one of the compression and the expansion ratios is given the incoherent value, the equalizer section is put in an abnormal condition in which the equalization operation is not exactly carried out. However, it is extremely difficult to recover the equalizer section from the abnormal condition to a normal condition in which the equalization operation is exactly carried out.